Recently, it has been desired that a vacuum pump requires a smaller energy (electrical power) to reduce CO2 emission in view of an environmental control. In Europe (EC), a chemical gas vacuum pump needs to have a discharge side temperature not more than 135° C. according to a safety standard. The temperature corresponds to a temperature grade T4 of the standard, in which acetaldehyde, trimethylamine, ethyl-methyl-ether, diethyl-ether, etc. are listed. These materials need to have a gas temperature not more than 135° C. at an outer surface thereof.
A conventional screw vacuum pump is disclosed in Japanese Patent Application Laid-open No. 63-36085, which is a single-stage pump having a pair of screw rotors. Another conventional screw vacuum pump is shown in FIG. 5, which is a two-stage pump having a pair of screw rotors.
This vacuum pump 61 has a pair of left and right screw rotors 63, 64 rotatively engaged with each other in a casing 62. The screw rotor 63 rotates clockwise while the screw rotor 64 rotates counterclockwise. Each of the rotors 63, 64 has helical teeth 65, 66 being different from each other in pitch. The helical tooth 65 has a larger pitch and is located in the side of a suction port 67 defined in the casing 62, while the helical tooth 66 having a smaller pitch is located in the side of a delivery port (not shown) of the casing 62.
Each of the screw rotors 63, 64 is supported at each axial end thereof by a bearing 73 or 68. The screw rotors 63, 64 can rotate adversely relative to each other via a timing gear 69 located at one end of thereof. A rotor shaft 70 couples operatively to a drive motor.
The rotation of the screw rotors 63, 64 compresses a gas introduced into a chamber 71 located in the side of the first helical teeth 65 from the suction port 67. The compressed gas is transferred into a chamber 72 of the second helical teeth 66, and the gas is further compressed in the chamber 72 to discharge it from the delivery port under an atmospheric pressure.
However, the conventional vacuum pump 61, as illustrated in a characteristic curve of FIG. 6, requires a comparatively larger power (shat driving power La), which disadvantageously increases a delivery gas temperature more than 200° C. In FIG. 6, a lower graph shows a shaft driving power (kW) while an upper graph shows a discharge gas flow rate (1/minute). Horizontal coordinates correspond to vacuum degrees (MPaA). Moreover, the gas compressed via the two stages tends to cause a considerable pressure loss due to a gas leak through a gap between the pair of screw rotors 63, 64. This undesirably decreases a discharge gas flow rate S as shown in an upper one of graphs of FIG. 6.
Screw vacuum pumps having such a property not only require a larger motor power but also compress a gas undesirably to create a gas temperature more than 135° C. Particularly, the screw vacuum pumps take a longer discharging time when a gas is compressed from a vacuum to an atmospheric pressure, which is an undesirable performance of them.
In view of the aforementioned situation, an object of the invention is to provide a vacuum pump requiring a reduced power and enhancing a reduced CO2 emission. The vacuum pump has an inside gas temperature (a temperature in a delivery side) that fulfills an EN standard (not more than 135° C.). The vacuum pump is improved in safety and its gas delivery performance.